Method and system for crest factor reduction

ABSTRACT

Methods and systems for crest factor reduction may comprise generating an original waveform, generating a distortion signal by reducing a crest factor of the original waveform, generating an error signal by subtracting out the original waveform from the distortion signal, generating a conditioned waveform by adding the error signal to the original waveform, and amplifying the conditioned waveform. The crest factor of the original waveform may be reduced based on spectral mask requirements. The crest factor of the original waveform may be reduced using a limiter. The power amplifier may comprise a programmable gain amplifier (PGA). The distortion signal may be generated based on a PGA model and/or a predistortion model. A signal from an output of the PA may be fed back to the PGA model. The PGA model may be dynamically configured. The crest factor of the original waveform may be reduced in an analog domain and/or a digital domain. The error signal may be filtered utilizing a distortion shaping filter.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

This application makes reference to and claims priority to U.S.Provisional Application Ser. No. 61/923,338 filed on Jan. 3, 2014. Theabove identified application is hereby incorporated herein by referencein its entirety.

FIELD

Certain embodiments of the disclosure relate to communication. Morespecifically, certain embodiments of the disclosure relate to a methodand system for crest factor reduction.

BACKGROUND

Conventional methods of crest factor reduction can be inefficient and/orineffective. Further limitations and disadvantages of conventional andtraditional approaches will become apparent to one of skill in the art,through comparison of such systems with some aspects of the presentinvention as set forth in the remainder of the present application withreference to the drawings.

BRIEF SUMMARY

A system and/or method for crest factor reduction substantially as shownin and/or described in connection with at least one of the figures, asset forth more completely in the claims.

Various advantages, aspects and novel features of the presentdisclosure, as well as details of an illustrated embodiment thereof,will be more fully understood from the following description anddrawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a diagram of an exemplary communication device with crestfactor reduction, in accordance with an example embodiment of thedisclosure.

FIG. 1B is a block diagram of an example transmitter operable to performcrest factor reduction in accordance with aspects of the disclosure.

FIG. 2A is a block diagram of an example implementation of the crestfactor reduction module of the transmitter of FIG. 1B.

FIG. 2B is a block diagram of an example implementation of the crestfactor reduction module of the transmitter of FIG. 1B.

FIG. 3A is a block diagram of a transmitter operable to perform crestfactor reduction using a dynamically configured power amplifier model.

FIG. 3B is a block diagram of another transmitter operable to performcrest factor reduction using a dynamically configured power amplifiermodel.

FIGS. 4A and 4B are block diagrams of an example transmitter operable toperform crest factor reduction and predistortion in accordance withaspects of this disclosure.

DETAILED DESCRIPTION

Certain aspects of the disclosure may be found in a method and systemfor crest factor reduction. Exemplary aspects may comprise generating anoriginal waveform, generating a distortion signal by reducing a crestfactor of the original waveform, generating an error signal bysubtracting out the original waveform from the distortion signal,generating a conditioned waveform by adding the error signal to theoriginal waveform, and amplifying the conditioned waveform. The crestfactor of the original waveform may be reduced based on spectral maskrequirements. The crest factor of the original waveform may be reducedusing a limiter. The power amplifier may comprise a programmable gainamplifier (PGA). The distortion signal may be generated based on a PGAmodel and/or a predistortion model. A signal from an output of the PAmay be fed back to the PGA model. The PGA model may be dynamicallyconfigured. The crest factor of the original waveform may be reduced inan analog domain and/or a digital domain. The error signal may befiltered utilizing a distortion shaping filter.

As utilized herein the terms “circuits” and “circuitry” refer tophysical electronic components (i.e. hardware) and any software and/orfirmware (“code”) which may configure the hardware, be executed by thehardware, and or otherwise be associated with the hardware. As usedherein, for example, a particular processor and memory may comprise afirst “circuit” when executing a first one or more lines of code and maycomprise a second “circuit” when executing a second one or more lines ofcode. As utilized herein, “and/or” means any one or more of the items inthe list joined by “and/or”. As an example, “x and/or y” means anyelement of the three-element set {(x), (y), (x, y)}. In other words, “xand/or y” means “one or both of x and y”. As another example, “x, y,and/or z” means any element of the seven-element set {(x), (y), (z), (x,y), (x, z), (y, z), (x, y, z)}. In other words, “x, y and/or z” means“one or more of x, y and z”. As utilized herein, the term “exemplary”means serving as a non-limiting example, instance, or illustration. Asutilized herein, the terms “e.g.,” and “for example” set off lists ofone or more non-limiting examples, instances, or illustrations. Asutilized herein, circuitry is “operable” to perform a function wheneverthe circuitry comprises the necessary hardware and code (if any isnecessary) to perform the function, regardless of whether performance ofthe function is disabled or not enabled (e.g., by a user-configurablesetting, factory trim, etc.).

FIG. 1A is a diagram of an exemplary communication device with crestfactor reduction, in accordance with an example embodiment of thedisclosure. Referring to FIG. 1A, there is shown a transmitter 101 chipcomprising a radio frequency (RF) module 105, a digital to analogconverter (DAC) 107, a digital front end (DFE) 113, a memory 115, and aprocessor 117. In an example scenario, the transmitter chip comprises asingle CMOS chip. In another example scenario, the transmitter chipcomprises a plurality of chips.

The transmitter 101 may be in a transceiver, for example, and may beutilized for transmitting and receiving RF signals carrying multiplechannels of data desired and/or generated by a user or service provider.In an example scenario, the transmitter 101 may comprise a wirelesscommunication device. While the example embodiment shown in FIG. 1Ashows crest factor reduction in an RF transmission scenario, it mayapply to any case where crest factor may impact amplifier performance.

The RF module 105 may comprise one or more RF receive (Rx) and transmit(Tx) paths for receiving and transmitting RF signals. The RF module 105may comprise impedance matching elements, LNAs, power amplifiers,variable gain amplifiers, and filters, for example. The RF module 105may thus be operable to transmit, receive, amplify, and filter RFsignals before communicating them to an antenna for transmission. In anexample scenario, the RF module 105 may comprise crest factor reductionmodule 109, which may be operable to reduce the crest factor, i.e., thepeak magnitude to average value, of a signal to be communicated as RFOUT 117. This may be achieved by limiting the magnitude of a signal,such as with a limiter, for example, which may distort the signal, whichmay be compensated for by applying a distortion signal to thereduced-crest-factor signal. The limiting function may be of“hard-limiting” or “soft-limiting” nature. Soft-limiting is by means ofa non-linear compressing function, while the output in hard-limiting islinear until it reaches the maximum and is clamped at that value.Soft-limiting and hard-limiting are illustrated by the insets in FIG.2B.

The application of the compensating distortion is in effect filteringthe distortion generated by limiting the signal.

The DAC 107 may comprise a wideband and/or time-interleaved ADC and maybe operable to convert digital signals received from the DFE 113 toanalog signals. In an example scenario, the DAC 107 may comprise adirect-sampled ADC and may comprise two parallel ADC paths, eachsampling 180 degrees out of phase with each other, resulting in a totalsampling rate that is double that of each individual path. For example,if each ADC path samples at 2.5 GHz, the resulting sampling rate for thesignal communicated to the digital front end 113 is 5.0 GHz.

The digital front end 113 may comprise circuitry for receiving a datastream from the processor 117 and for communicating samples to the DAC107. The processor 117 may comprise a general purpose processor, such asa reduced instruction set computing (RISC) processor, for example, thatmay be operable to control the functions of the transmitter 101. Forexample, the processor 117 may configure the switches 109 in an open orclosed position. Additionally, the processor 117 may demodulate basebandsignals received from the digital front end 113.

The memory 115 may comprise a programmable memory module that may beoperable to store software and data, for example, for the operation ofthe transmitter 101. Furthermore, the memory 115 may store distortionfiltering settings for the crest factor reduction module 109.

Various aspects of this disclosure may be applied to improve poweramplifier performance while OFDM or multi-channel signals are beingtransmitted. This may include crest factor reduction to suppress highpeaks, avoid catastrophic clipping in the power amplifier PA, and shapespectral regrowth away from problematic frequencies.

FIG. 1B depicts a block diagram of a transmitter operable to performcrest factor reduction in accordance with aspects of this disclosure.The transmitter 100 comprises baseband processing circuit 102, digitalto analog converter 104, crest factor reduction (CFR) module 106, and aprogrammable gain power amplifier (PGA) 108. While the crest factorreduction circuit 102 is shown between the DAC 104 and the PGA 108, itmay instead be implemented partially or wholly in the digital domainprior to the DAC 104, such as by being integrated in the basebandprocessing circuit 102.

The baseband processing circuit 102 may perform such functions as, forexample, filtering, forward error correction encoding, bit-to-symbolmapping, digital upconversion, and/or the like, and may generate asignal 103. The signal 103 output by baseband processing circuit 102 maythen be converted to an analog representation 105 by the DAC 104. Theanalog signal 105 may then be processed by CFR 106 to generate signal107, with the goal of the processing being such that the crest factor ofthe signal 107 is less than the crest factor of the signal 105. Thesignal 107 may then be amplified by the PGA 108 for transmission ontothe physical medium (e.g., wire, optical fiber, or air).

In an example scenario, the CFR module 106 may reduce the crest factorof the analog signal 105, which may be accomplished with a limiter, forexample. This limiting of the received signal 105 may in turn generatein-band and/or out-of-band distortion, i.e., an error signal, which maybe compensated by the CFR module 106 by subtracting out a compensatingerror signal generated by the CFR module 106.

The CFR module 106 may comprise a distortion shaping filter that may beconfigured to eliminate and/or reduce out-of-band distortion resultingfrom the reduction of the crest factor of the received signal. Inaddition, the compensating error signal may be generated in the CFRmodule 106 by considering at least three factors comprising thepeak-to-average ratio (crest factor), the in-band distortion, and theout-of-band distortion. The factors may be determined by spectral maskrequirements for the transmitter.

FIG. 2A depicts a block diagram of an example implementation of thecrest factor reduction module of the transmitter of FIG. 1B for example.The example implementation of CFR module 106 shown in FIG. 2A comprisesa circuit 202 which is a model of the PGA 108, a combiner 204, adistortion shaping filter 206, and a combiner 208. The output 105 of theDAC (or the DFE, for example, if in the digital domain) may be input tothe circuit 202 to generate a distortion signal 203 which models thesignal that will be output by the PGA 108. The signal 105 may then besubtracted off of the distortion signal 203 by the combiner 204,resulting in the error signal 205 which represents the distortionintroduced by the PGA model 202 (which, in turn, represents thedistortion that will be introduced by the actual PGA 108).

The distortion shaping filter 206 may be operable to analyze the signal205 to determine where spectral regrowth is arising due to the PGA model202. Such regrowth could be in-band (i.e., within the frequency bandallocated for communications by the transmitter 100), in an adjacentband (i.e., relatively close to the allocated frequency band), and/or ata harmonic of the frequency band allocated for communications by thetransmitter 100 (e.g., much higher or lower in frequency than theadjacent bands). The distortion shaping filter 206 may then configureitself to shape the energy of the error signal 205 (i.e., the energy ofthe predicted distortion in the absence of circuit 206) such that whenthe resulting filtered error signal 207 is combined with the signal 105,the resulting signal, the conditioned waveform 107, has a lower crestfactor than the signal 105. For example, the signal 207 may be 180° outof phase with the signal 105 such that cancellation occurs in combiner208.

The CFR module 106 may operate at a slower rate than the DAC samplerate. For example, the distortion shaping filter 206 may update everyNth (an integer) group of M (an integer) samples of the signal 105 or,in an OFDM system, it may update at the OFDM-symbol rate. Because theCFR module 106 may operate below the DAC sampling frequency, the CFRmodule 106 may iterate (as indicated by line 210) on a particular sampleor group of samples to refine the results.

It may be the case that the PA 108 is predicted to introduce a lot ofdistortion in one or more of three frequency regions (the three regionsbeing in-band, adjacent bands, and harmonic bands) while introducinglittle or no distortion in other(s) of the regions. Accordingly, thefiltering may trade off margin in one region (e.g., in-band) to gainmargin in another (e.g., harmonic bands). The selected trade-off maychange with each update of the filter 206 (e.g., the trade-off may bedetermined on a symbol-by-symbol basis).

FIG. 2B depicts a block diagram of an example implementation of thecrest factor reduction module of the transmitter of FIG. 1B. FIG. 2Bdepicts a particular implementation where the PGA model 202 simplymodels the PGA 108 as clipping above a threshold (which may beprogrammable). The limiter 202 may therefore clip the output level suchthat the crest factor of the signal is reduced. The limiting function ofthe limiter 202 may be of hard-limiting type or soft-limiting type, asillustrated in the insets of FIG. 2B.

However, this clipping leads to distortion in the signal that may thenbe compensated for with the distortion shaping filter 206, similar tothe process described with respect to FIG. 2A.

The distortion shaping filter 206 may be operable to analyze the signal205 to determine where spectral regrowth is arising due to the clippingfrom the limiter 202. The distortion shaping filter 206 may thenconfigure itself to shape the energy of the error signal 205 such thatwhen the resulting filtered error signal 207 is combined with the signal105, the resulting signal, conditioned waveform 107, has a lower crestfactor than the signal 105. For example, the signal 207 may be 180° outof phase with the signal 105 such that cancellation occurs in combiner208. One benefit of the CFR module 106 of FIGS. 2A and 2B is that it isnot necessary to determine the inverse model of the PGA 108.

FIG. 3A is a block diagram of a transmitter operable to perform crestfactor reduction using a dynamically configured power amplifier model.The transmitter 300 is similar to the transmitter 100 of FIG. 1B, butadditionally comprises a circuit 302 that may use feedback todynamically configure the PGA model 202. The behavior of the PGA 108 maychange based on the signals being processed, temperature, and/or otherfactors. Accordingly, to accurately predict the distortion that theamplifier 108 will introduce, the model 202 may be adapted so that itaccurately models the PGA 108 under different conditions.

In an example implementation, the circuit 302 may sense/track the 1 dBcompression point of the PGA 108 and update the PGA model 202accordingly. In an example implementation, the compression point may bemonitored directly or a proxy for the compression point may be used. Forexample, efficiency of the PGA 108 may be used as a proxy for thecompression point and may be determined by measuring strength, e.g., thereceived signal strength indication (RSSI) of the signal 109 and DCcurrent drawn by the PGA 108. This measured efficiency may be input topthe PGA model 202 to dynamically configure the PGA model 202 andtherefore dynamically control the in-band and out-of-band distortioncompensation.

FIG. 3B is a block diagram of another transmitter operable to performcrest factor reduction using a dynamically configured power amplifiermodel. The transmitter 310 is similar to the transmitter 300 butillustrates the CFR module 106 in the digital domain, i.e., prior to theDAC 104. In this manner, digital processing techniques may be utilizedto configure the CFR reduction and distortion compensation. In thisinstance, the circuit 302 may then also comprise an analog-to-digitalconverter (ADC) to convert the analog signal at the output of the PGA109 to digital for processing in the digital domain.

As with the transmitter 300, the transmitter 310 may use feedback todynamically configure the PGA model 202. The behavior of the PGA 108 maychange based on the signals being processed, temperature, and/or otherfactors. Accordingly, to accurately predict the distortion that theamplifier 108 will introduce, the model 202 may be adapted so that itaccurately models the PGA 108 under different conditions.

FIGS. 4A and 4B are block diagrams of a transmitter operable to performcrest factor reduction and predistortion in accordance with aspects ofthe present disclosure. The transmitter 400 is similar to thetransmitter 100 of FIG. 1B but additionally comprises a predistortioncircuit 402 which may be operable to further compensate for distortionintroduced by the PGA 108. In such an example implementation, the PGAmodel 202 of FIG. 2A may be replaced by a PGA+predistortion model 404,as shown in FIG. 4B, so that the CFR module 106 can account for thepresence of the predistortion 402.

In an example implementation, the CFR module 106 may mostly have animpact when the signal levels are relatively high. In such animplementation, the predistortion circuit 402 may be operable to accountfor such operation of the CFR module 106 by only predistorting signalsthat are relatively low in amplitude (e.g., below a predeterminedthreshold). This may enable the CFR module 106 and predistortion circuit402 to work in harmony to achieve better overall system performance.

As with previous examples, the CFR 106 shown in FIG. 4B may beimplemented in the analog domain, the digital domain, or a combinationof analog and digital. By incorporating both PGA and predistortionmodels, higher performance may be achieved in the PGA.

In an embodiment of the disclosure, a method and system for crest factorreduction may comprise generating an original waveform, generating adistortion signal by reducing a crest factor of the original waveform,generating an error signal by subtracting out the original waveform fromthe distortion signal, generating a conditioned waveform by adding theerror signal to the original waveform, and amplifying the conditionedwaveform. The crest factor of the original waveform may be reduced basedon spectral mask requirements.

The crest factor of the original waveform may be reduced using alimiter. The power amplifier may comprise a programmable gain amplifier(PGA). The distortion signal may be generated based on a PGA modeland/or a predistortion model. A signal from an output of the PA may befed back to the PGA model. The PGA model may be dynamically configured.The crest factor of the original waveform may be reduced in an analogdomain and/or a digital domain. The error signal may be filteredutilizing a distortion shaping filter.

Other embodiments of the disclosure may provide a non-transitorycomputer readable medium and/or storage medium, and/or a non-transitorymachine readable medium and/or storage medium, having stored thereon, amachine code and/or a computer program having at least one code sectionexecutable by a machine and/or a computer, thereby causing the machineand/or computer to perform the steps as described herein for crestfactor reduction.

Accordingly, aspects of the disclosure may be realized in hardware,software, firmware or a combination thereof. The disclosure may berealized in a centralized fashion in at least one computer system or ina distributed fashion where different elements are spread across severalinterconnected computer systems. Any kind of computer system or otherapparatus adapted for carrying out the methods described herein issuited. A typical combination of hardware, software and firmware may bea general-purpose computer system with a computer program that, whenbeing loaded and executed, controls the computer system such that itcarries out the methods described herein.

One embodiment of the present disclosure may be implemented as a boardlevel product, as a single chip, application specific integrated circuit(ASIC), or with varying levels integrated on a single chip with otherportions of the system as separate components. The degree of integrationof the system will primarily be determined by speed and costconsiderations. Because of the sophisticated nature of modernprocessors, it is possible to utilize a commercially availableprocessor, which may be implemented external to an ASIC implementationof the present system. Alternatively, if the processor is available asan ASIC core or logic block, then the commercially available processormay be implemented as part of an ASIC device with various functionsimplemented as firmware.

The present disclosure may also be embedded in a computer programproduct, which comprises all the features enabling the implementation ofthe methods described herein, and which when loaded in a computer systemis able to carry out these methods. Computer program in the presentcontext may mean, for example, any expression, in any language, code ornotation, of a set of instructions intended to cause a system having aninformation processing capability to perform a particular functioneither directly or after either or both of the following: a) conversionto another language, code or notation; b) reproduction in a differentmaterial form. However, other meanings of computer program within theunderstanding of those skilled in the art are also contemplated by thepresent disclosure.

While the disclosure has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the present disclosure. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the present disclosure without departing from itsscope. Therefore, it is intended that the present disclosure not belimited to the particular embodiments disclosed, but that the presentdisclosure will include all embodiments falling within the scope of theappended claims.

What is claimed is:
 1. A method for communication, the methodcomprising: generating an original waveform to be amplified by aprogrammable gain amplifier (PGA); generating a distortion signalutilizing a PGA model that predicts an output signal of the PGA based onthe original waveform; generating an error signal by subtracting out theoriginal waveform from the distortion signal; filtering the generatederror signal; generating a conditioned waveform with crest factorreduced from that of the original waveform by adding the filtered errorsignal to the original waveform; and amplifying the conditionedwaveform.
 2. The method according to claim 1, comprising reducing thecrest factor of the original waveform based on spectral maskrequirements.
 3. The method according to claim 1, comprising iteratingthe generating of the conditioned waveform.
 4. The method according toclaim 1, comprising generating the distortion signal based on apredistortion model.
 5. The method according to claim 4, comprisingfeeding back a signal from an output of the PGA to the PGA model.
 6. Themethod according to claim 4, comprising dynamically configuring the PGAmodel.
 7. The method according to claim 1, comprising reducing the crestfactor of the original waveform in an analog domain.
 8. The methodaccording to claim 1, comprising reducing the crest factor of theoriginal waveform in a digital domain.
 9. The method according to claim1, comprising filtering the error signal utilizing a distortion shapingfilter.
 10. A system for communication, the system comprising: one ormore circuits on a chip comprising a transmitter, said one or morecircuits operable to: generate a signal for amplification by aprogrammable gain amplifier (PGA) and subsequent transmission; generatea distortion signal utilizing a PGA model that predicts an output signalof the PGA based on the signal for amplification; generate an errorsignal by subtracting the signal for amplification from the distortionsignal; filter the generated error signal; generate a conditionedwaveform with crest factor reduced from that of the signal foramplification by adding the filtered error signal to the signal foramplification; and amplify the conditioned waveform.
 11. The systemaccording to claim 10, wherein the one or more circuits are operable toreduce the crest factor of the signal for amplification based onspectral mask requirements.
 12. The system according to claim 10,wherein said one or more circuits are operable to iterate the generatingof the conditioned waveform.
 13. The system according to claim 10,wherein said one or more circuits are operable to generate thedistortion signal based on a predistortion model.
 14. The systemaccording to claim 13, wherein said one or more circuits are operable tofeed back a signal from an output of the PGA to the PGA model.
 15. Thesystem according to claim 13, wherein said one or more circuits areoperable to dynamically configure the PGA model.
 16. The systemaccording to claim 10, wherein said one or more circuits are operable toreduce the crest factor of the signal for amplification in an analogdomain.
 17. The system according to claim 10, wherein said one or morecircuits are operable to filter the error signal utilizing a distortionshaping filter.
 18. A method for communication, the method comprising:generating an original waveform to be amplified by a programmable gainamplifier (PGA); generating a distortion signal utilizing a PGA modelthat predicts an output signal of the PGA based on the originalwaveform; generating an error signal by subtracting out the originalwaveform from the distortion signal; filtering the error signalutilizing a distortion shaping filter; generating a conditioned waveformwith crest factor reduced from that of the original waveform by addingthe filtered error signal to the original waveform; and amplifying theconditioned waveform.